EC441 Fall 2018 Introduction to Computer Networking Chapter4: Network Layer Data Plane

Transcription

1 EC441 Fall 2018 Introduction to Computer Networking Chapter4: Network Layer Data Plane This presentation is adapted from slides produced by Jim Kurose and Keith Ross for their book, Computer Networking: A Top Down Approach and is used with permission of the authors as expressed on the original PowerPoint documents That material is copyright J.F Kurose and K.W. Ross, All Rights Reserved Slides produced by Prof Carruthers are marked as such. All other slides are adapted from or the same as the originals by Kurose and Ross. EC441 Ch4-1

2 Chapter 4: Outline Network layer overview: forwarding and routing Internet service model Addressing: IP, CIDR, DHCP, NAT IPv4 datagram format Fragmentation and reassembly IPv6 Note: the material on routers and software defined networking (SDN) will be covered later in the semester. EC441 Ch4-2

3 Network layer transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router router examines header fields in all IP datagrams passing through it application transport network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical EC441 Ch4 -!3

4 Two key network-layer functions network-layer functions: forwarding: move packets from router s input to appropriate router output routing: determine route taken by packets from source to destination routing algorithms analogy: taking a trip forwarding: process of getting through single interchange routing: process of planning trip from source to destination EC441 Ch4 -!4

5 Network layer: data plane, control plane Data plane local, per-router function determines how datagram arriving on router input port is forwarded to router output port forwarding function values in arriving packet header Control plane network-wide logic determines how datagram is routed among routers along end-end path from source host to destination host two control-plane approaches: traditional routing algorithms: implemented in routers software-defined networking (SDN): implemented in (remote) servers EC441 Ch4 -!5

6 IP Service Model Connectionless (datagram-based) Best-effort delivery (unreliable service) packets are lost packets are delivered out of order duplicate copies of a packet are delivered packets can be delayed for a long time EC441 Ch4-6

7 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP Network layer Routing protocols path selection RIP, OSPF, BGP routing table IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling Link layer physical layer EC441 Ch4-7

8 IP Internet Concatenation of Networks Network 1 (Ethernet) H7 R3 H8 H1 H2 H3 Network 2 (Ethernet) R1 Network 4 (point-to-point) Protocol Stack H4 Network 3 (FDDI) R2 H5 H6 H1 H8 TCP R1 R2 R3 TCP IP IP IP IP IP ETH ETH FDDI FDDI PPP PPP ETH ETH EC441 Ch4-8

9 Datagram Forwarding Strategy every datagram contains destination s address if directly connected to destination network, then forward to host if not directly connected to destination network, then forward to some router forwarding table maps network number into next hop each host has a default router each router maintains a forwarding table Example (R2) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0 EC441 Ch4-9

10 IP addressing: introduction IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router s typically have multiple interfaces host typically has one or two interfaces (e.g., wired Ethernet, wireless ) IP addresses associated with each interface = EC441 Ch4 -!10

11 Global Addresses Properties globally unique hierarchical: network + host Dot Notation A: B: Network Host Network Host C: Network Host EC441 Ch4-11

12 IP addressing: CIDR CIDR: Classless InterDomain Routing subnet portion of address of arbitrary length address format: a.b.c.d/x, where x is # bits in subnet portion of address subnet part host part /23 EC441 Ch4-12

13 Subnets IP address: subnet part (high order bits) host part (low order bits) What s a subnet? device interfaces with same subnet part of IP address can physically reach each other without intervening router subnet network consisting of 3 subnets EC441 Ch4-13

14 Subnets / /24 Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet /24 Subnet mask: /24 EC441 Ch4-14

15 Datagram forwarding (2) All hosts on a subnet have same subnet mask Subnet # = IP & subnet mask Forwarding: Check subnet # of dest, forward to a router if on different subnet Subnet mask: Subnet number: H R Subnet mask: Subnet number: Subnet # Subnet mask Next hop Int Int R2 H R H Routing Table, R1 Subnet mask: Subnet number: EC441 Ch4-15

16 IP addresses: how to get one? Q: how does network get subnet part of IP addr? A: gets allocated portion of its provider ISP s address space ISP's block /20 Organization /23 Organization /23 Organization / Organization /23 EC441 Ch4 -!16

17 Hierarchical addressing: route aggregation hierarchical addressing allows efficient advertisement of routing information: Organization /23 Organization /23 Organization /23 Organization /23. Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet ISPs-R-Us Send me anything with addresses beginning /16 EC441 Ch4 -!17

18 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization /23 Organization /23 Organization /23. Fly-By-Night-ISP Send me anything with addresses beginning /20 Internet Organization /23 ISPs-R-Us Send me anything with addresses beginning /16 or /23 EC441 Ch4 -!18

19 Longest prefix matching When looking for forwarding table entry for given destination address, use longest address prefix that matches destination address. Destination Address Range *** ********* ********* *** ********* otherwise Link interface EC441 Ch4-19

20 IP addresses: how to get one? Q: How does a host get IP address? Static IP address hard-coded by system admin in a file DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server plug-and-play EC441 Ch4-20

21 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network can renew its lease on address in use allows reuse of addresses (only hold address while connected/ on ) support for mobile users who want to join network (more shortly) DHCP overview: host broadcasts DHCP discover msg [optional] DHCP server responds with DHCP offer msg [optional] host requests IP address: DHCP request msg DHCP server sends address: DHCP ack msg EC441 Ch4-21

22 DHCP client-server scenario / DHCP server arriving DHCP client needs address in this network / /24 EC441 Ch4-22

23 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 arriving client DHCP request src: , 68 dest:: , 67 yiaddr: transaction ID: 655 lifetime: 3600 secs DHCP offer src: , 67 dest: , 68 yiaddr: transaction ID: 654 lifetime: 3600 secs DHCP ACK src: , 67 dest: , 68 yiaddr: transaction ID: 655 lifetime: 3600 secs EC441 Ch4-23

24 DHCP: more than IP addresses DHCP can return more than just allocated IP address on subnet: address of first-hop router for client name and IP address of DNS server network mask (indicating network versus host portion of address) EC441 Ch4-24

25 NAT: network address translation 2: NAT router changes datagram source addr from , 3345 to , 5001, updates table 2 NAT translation table WAN side addr LAN side addr , , 3345 S: , 5001 D: , S: , 80 D: , : reply arrives dest. address: , S: , 3345 D: , 80 1 S: , 80 D: , : host sends datagram to , : NAT router changes datagram dest addr from , 5001 to , 3345 * Check out the online interactive exercises for more examples: EC441 Ch4 -!25

26 NAT: Network Address Translation rest of Internet local network (e.g., home network) / All datagrams leaving local network have same single source NAT IP address: , different source port numbers Datagrams with source or destination in this network have /24 address for source, destination (as usual) EC441 Ch4-26

27 NAT: network address translation 16-bit port-number field: 60,000 simultaneous connections with a single LANside address! NAT is controversial: routers should only process up to layer 3 address shortage should be solved by IPv6 violates end-to-end argument NAT possibility must be taken into account by app designers, e.g., P2P applications NAT traversal: what if client wants to connect to server behind NAT? EC441 Ch4 -!27

28 IP addressing: the last word... Q: how does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes EC441 Ch4 -!28

29 IP datagram format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. len 16-bit identifier time to live type of service upper layer 32 bits flgs length fragment offset header checksum 32 bit source IP address 32 bit destination IP address options (if any) data (variable length, typically a TCP or UDP segment) total datagram length (bytes) for fragmentation/ reassembly e.g. timestamp, record route taken, specify list of routers to visit. EC441 Ch4 -!29

30 Datagram Format - Summary Version HLen TOS Length Ident Flags Offset TTL Protocol Checksum SourceAddr DestinationAddr Options (variable) Pad (variable) Data EC441 Ch4-30

31 IP fragmentation, reassembly network links have MTU (max.transfer size) - largest possible link-level frame different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams EC441 Ch4 -!31

32 IP fragmentation, reassembly example: 4000 byte datagram MTU = 1500 bytes length =4000 ID =x fragflag =0 offset =0 one large datagram becomes several smaller datagrams 1480 bytes in data field length =1500 ID =x fragflag =1 offset =0 offset = 1480/8 length =1500 ID =x fragflag =1 offset =185 length =1040 ID =x fragflag =0 offset =370 EC441 Ch4 -!32

33 IPv6: motivation initial motivation: 32-bit address space soon to be completely allocated. additional motivation: header format helps speed processing/forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed EC441 Ch4 -!33

34 IPv6 datagram format priority: identify priority among datagrams in flow flow Label: identify datagrams in same flow. (concept of flow not well defined). next header: identify upper layer protocol for data ver pri flow label payload len next hdr hop limit source address (128 bits) destination address (128 bits) data 32 bits EC441 Ch4 -!34

35 Other changes from IPv4 checksum: removed entirely to reduce processing time at each hop options: allowed, but outside of header, indicated by Next Header field ICMPv6: new version of ICMP additional message types, e.g. Packet Too Big multicast group management functions EC441 Ch4 -!35

36 Transition from IPv4 to IPv6 not all routers can be upgraded simultaneously no flag days how will network operate with mixed IPv4 and IPv6 routers? tunneling: IPv6 datagram carried as payload in IPv4 datagram among IPv4 routers IPv4 header fields IPv4 source, dest addr IPv6 header fields IPv6 source dest addr UDP/TCP payload IPv4 payload IPv6 datagram IPv4 datagram EC441 Ch4 -!36

37 Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 EC441 Ch4 -!37

38 Tunneling logical view: A IPv6 B IPv6 IPv4 tunnel connecting IPv6 routers E IPv6 F IPv6 physical view: A B C D E F IPv6 IPv6 IPv4 IPv4 IPv6 IPv6 flow: X src: A dest: F data src:b dest: E Flow: X Src: A Dest: F src:b dest: E Flow: X Src: A Dest: F flow: X src: A dest: F data data data A-to-B: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4 E-to-F: IPv6 EC441 Ch4 -!38

39 IPv6: adoption Google: 8% of clients access services via IPv6 NIST: 1/3 of all US government domains are IPv6 capable Long (long!) time for deployment, use 20 years and counting! think of application-level changes in last 20 years: WWW, Facebook, streaming media, Skype, Why? EC441 Ch4 -!39

40 Chapter 4: done! Question: how are forwarding tables (destination-based forwarding) or flow tables (generalized forwarding) computed? Answer: by the control plane (next chapter) EC441 Ch4 -!40